The present invention relates to an oscillating gripper unit for printing machines, operable to transfer sheets to be printed from a table, on which the sheets are stacked against appropriate abutments, to the surface of a rotating cylinder; the invention further relates to a process making use of this gripper unit to effect the transfer.
It is known to obtain transfer of sheets from the support table to a rotating cylinder (print cylinder or pressure cylinder) of a printing machine by means of oscillating gripper units including a rocker element carrying an oscillating gripper member and a gripper foot with which the gripper member cooperates to grip the sheet; the foot is controlled indirectly by an associated control shaft which adjusts the foot's position with respect to the gripper member to adapt the grippers to sheets of different thickness and the position of the axis of oscillation of the entire gripper unit is varied with respect to that of the axis of the rotating cylinder in the return stroke, that is during the movement of the gripper from the rotating cylinder towards the table; in particular, the entire gripper unit is raised with respect to the rotating cylinder by means of suitable mechanisms constituted by crank mechanisms or eccentrics in order to avoid interference of the gripper member and the associated foot with the surface of the rotating cylinder, such interference being avoided in the forward stroke from the table to the cylinder, in which the transport of the sheet takes place, by the presence of a cavity on the surface of the rotating cylinder, in which are housed sheet-conveyor grippers of the cylinder itself. Proceeding in this way is not entirely free from disadvantages; in fact, if the raising of the gripper unit is obtained by means of a crank mechanism, given the high mass in movement there are significant problems in lubrication; on the other hand, if the gripper unit is displaced by mounting it on rotating eccentrics it is not always possible to obtain identical peripheral velocities of the rotating cylinder and the gripper, and a trajectory of the gripper which is perfectly tangential to the rotating cylinder upon exchange of the sheet.